1. Field of the Invention
The present invention relates to a transmission apparatus and communication apparatus having a power amplifier in which the operation mode is switched between saturation mode and non-saturation mode, and is suitable to a transmission apparatus and communication apparatus used in radio communication schemes including the W-CDMA (Wideband Code Division Multiple Access) scheme.
2. Description of the Related Art
Up till now, a transmission apparatus has been known in which current consumption can be reduced by operating the power amplifier in saturation mode provided in the tail of a transmission system. As a typical example, a polar modulation transmission apparatus has been proposed. The polar modulation transmission apparatus is disclosed in, for example, Patent Document 1 (U.S. Patent Application Laid-Open No. 2002/0177420).
FIG. 1 illustrates a schematic configuration of the polar modulation transmission apparatus disclosed in Patent Document 1. Here, the polar modulation technique is widely known and explanations thereof will be performed simply using FIG. 1. In the transmission apparatus of FIG. 1, transmission data is inputted to amplitude signal and phase signal forming section 11 and amplitude signal and phase signal forming section 11 forms an amplitude signal and phase signal according to transmission data.
Mixer 12 amplifies the amplitude signal according to the scaling coefficient from power control section 13. This amplified amplitude signal is supplied to the power supply terminal of power amplifier 14 as power supply voltage Vam of power amplifier 14. On the other hand, phase modulation section 15 performs up-conversion on the phase signal and acquires an RF phase signal. Here, phase modulation section 15 employs a configuration having a VCO (Voltage Controlled Oscillator) and PLL (Phase Locked Loop).
Amplitude adjustment circuit 16 adjusts the amplitude of the RF phase signal according to the scaling coefficient from power control section 13, and subsequently inputs the adjusted amplitude to the signal input terminal of power amplifier 14. Here, amplitude adjusting section 16 employs a configuration having an attenuator and VGA (Variable Gain Amplifier).
By this means, the apparatus in FIG. 1 combines the amplitude signal and the RF phase signal in power amplifier 14 and transmits the combined signal from an antenna (not shown).
By the way, according to the standard of transmission power control, a mobile phone employing the W-CDMA scheme needs to maintain the dynamic range of 74 dB with respect to the transmission power of the output signal outputted from power amplifier 14. To maintain such a wide dynamic range, the transmission apparatus shown in FIG. 1 is known to be provided such that the operation mode of power amplifier 14 is switched between saturation mode and non-saturation mode.
The operation of each mode will be explained using FIG. 2. The long and short dotted line in the figure shows the switching point between operation modes. When transmission power Pout is high, the operation is performed in saturation mode. In saturation mode, the transmission apparatus changes transmission power Pout by changing the level of power supply voltage Vam. Control of Vam alone does not make it possible to perform power control in a wide dynamic range of 74 dB, and, consequently, when transmission power Pout becomes smaller, the operation is performed in non-saturation mode. In non-saturation mode, the transmission apparatus changes the level of transmission power Pout by fixing the level of power supply voltage Vam and changing the level of power of RF phase modulation signal Ppm. Thus, the mode that changes transmission power Pout by changing power supply voltage Vam of power amplifier 14 is referred to as the “saturation mode,” and the mode that changes transmission power Pout by changing input signal power Ppm of power amplifier 14 is referred to as the “non-saturation mode.” The transmission apparatus maintains the dynamic range of 74 dB by combining these two modes.
However, the W-CDMA scheme performs simultaneous transmission and reception, and, in a mobile phone terminal employing the W-CDMA scheme, there is a problem of transmission output noise in the receiving band leaking into the receiving input terminal. Actually, when the transmission apparatus in FIG. 1 is mounted in a mobile phone terminal, the output terminal of the power amplifier is connected to an antenna via an antenna duplexer. Here, the input terminal of the receiving apparatus, in addition to the output terminal of the power amplifier, is also connected to this antenna duplexer. When this configuration is employed, there is a problem that transmission output noise, caused by the transmission apparatus, leaks into the receiving apparatus via the antenna duplexer.
Therefore, transmission output noise in the receiving band needs to be reduced to an extent where reception sensitivity is not degraded. To reduce noise from the output signal in FIG. 1, amplitude adjustment section 16 needs to be configured such that good noise characteristics are yielded.
For example, although amplitude adjustment section 16 may be comprised of a variable attenuator, when the variable attenuator is comprised of an active circuit such as a differential gain control amplifier, there is a problem of increased power consumption for yielding good noise characteristics. As another method, although the variable attenuator may be comprised of a passive circuit such as a π type resistance attenuator, in this case, there is a problem of increased power loss in the variable attenuator. Further, in the above-described cases, there is a problem that an excellent C/N (Carrier to Noise) ratio of the RF phase signal is degraded by the variable attenuator.